Acicular magnetic iron based alloy particles for magnetic recording and method of producing the same

ABSTRACT

Disclosed herein are acicular magnetic iron based alloy particles for magnetic recording, containing 1.5 to 10 mol % of B based on Fe (calculated as B) and 1.5 to 10 mol % of Co based on Fe (calculated as Co) in the vicinity of the surfaces of said particles and having a saturation magnetization of not less than 125 emu/g and an S.F.D. value of not more than 0.50, and a process for producing the same.

This is a continuation of application Ser. No. 07/388,025, filed Jul.31, 1989, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to acicular magnetic iron based alloyparticles for magnetic recording having high coercive force, highsaturation magnetization, excellent stability against oxidation andexcellent S.F.D. (Switching Field Distribution), and further having gooddispersibility ascribed to excellent wetting property anddeagglomeration property with respect to a binder, in particular, abinder containing a resin having an acid functional group, or a largespecific surface area, and a method of producing such particles.

With the recent development of smaller-sized and ligther-weight magneticrecording apparatuses, the necessity for a recording medium having ahigher performance such as a magnetic tape and a magnetic disk has beenincreasing more and more. In other words, a magnetic recording medium isrequired to have a higher recording density, higher outputcharacteristic, in particular, an improved frequency characteristic anda lowered noise level.

The properties in which magnetic particles are required to have in orderto satisfy the above-described demands on a magnetic recording medium,particularly, magnetic characteristics are a high coercive force and alarge saturation magnetization.

Magnetic particles suitable for high-output and high-density recording,namely, magnetic particles having a high coercive force and a largesaturation magnetization have recently been developed rapidly. Asmagnetic particles having such characteristics, acicular magnetic ironbased alloy particles obtained by heat-treating acicular goethiteparticles or acicular hematite particles in a reducing gas are known andput to practical use.

The coercive force of acicular magnetic iron based alloy particles isdependent on the shapes of the particles, in particular, to the aspectratio (major axis:minor axis) and has a tendency of aggrandizing withthe increase in the aspect ratio (major axis:minor axis). The saturationmagnetization has a tendency of increasing with the acceleration ofreduction by raising the reducing temperature. The coercive force andthe saturation magnetization have reverse correlation. That is, with theprogress of reduction, the saturation magnetization is enhanced but theshapes of the particles are collapsed, thereby reducing the coerciveforce. Thus, acicular magnetic iron based alloy particles having bothhigh coercive force and large saturation magnetization are stronglydemanded.

Acicular magnetic iron based alloy particles having a high coerciveforce and a large saturation magnetization are very unstable. Morespecifically, since the acicular magnetic iron based alloy particlesused for a magnetic recording medium are very fine particles having aparticle size of not more than 1 μm, the surface activity of theparticles is so large that when they are taken out into air afterreduction, they rapidly react with the oxygen in air to ignite andrelease heat. In addition, the oxidation reaction converts theseparticles into oxides, thereby greatly reducing the magneticcharacteristics, in particular, the saturation magnetization, so that itis impossible to obtain acicular magnetic iron based alloy particleshaving a large saturation magnetization which is an objective of thepresent invention. For these reasons, acicular magnetic iron based alloyparticles having excellent stability against oxidation are stronglydemanded.

There is no end to the recent demand for the improvement of theproperties of acicular magnetic iron based alloy particles. In additionto a high coercive force, a larger saturation magnetization, anexcellent stability against oxidation, and an excellent S.F.D.(Switching Field Distribution) is strongly demanded. This fact is clearfrom the descriptions in Japanese Patent Application Laid-Open (KOKAI)No. 63.26821 (1988): "FIG. 1 shows the relationship between the S.F.D.value, and the recording and reproducing output of the above-describedmagnetic disc. . . . The relationship between the S.F.D. value, and therecording and reproducing output is linear, as is obvious from FIG. 1,which proves that the use of ferromagnetic particles having a smallS.F.D. value enhances the recording and reproducing output. That is, inorder to increase the recording and reproducing output, the S.F.D. valueis preferably as small as possible. In order to obtain a higher outputmore than the ordinary one, it is necessary that the S.F.D. value is notmore than 0.6."

It is pointed out that a resin having an OH group, which hasconventionally been widely used as a resin for magnetic recording mediais disadvantageous in that the resin needs to contain a large amount ofdispersant in order to disperse magnetic particles in a binder and, as aresult, the dispersant remaining in the coated film has influence on thesurface properties of the coated film such as viscosity, which causes adefect in operation due to a change in the ambient temperature or thelike. To solve this problem, the recent tendency is to reduce the amountof dispersant mixed with a binder as much as possible or not used, andvarious investigations have been made on the kind of the resin, thesurface properties of magnetic particles, etc. As the resin, a resinhaving a COOH group or an SO₄ Na group has been put to use in place ofthe resin having an OH group, and as to the magnetic particles, theimprovement of the wetting property and the deagglomeration propertywith respect to the resin having an acid functional group such as a COOHgroup, an SO₄ Na group, etc. is strongly demanded in order to enhancethe dispersibility of a binder.

It is known that the noise level of a magnetic recording medium has atendency of lowering in proportion to the specific surface area ofacicular magnetic iron based alloy particles. This phenomenon is shownin, for example, "FIG. 1" of Japanese Patent Application Laid-Open(KOKAI) No. 58-159231 (1983).

"FIG. 1" shows the relationship between the noise level of a magnetictape produced from magnetic metal particles and the specific surfacearea of the particles used. The noise level linearly lowers with theincrease in the specific surface area of the particles. Acicularmagnetic iron based alloy particles are, therefore, strongly required tohave as large a specific surface area as possible.

Various attempts have heretofore been made to improve the properties ofacicular magnetic iron based alloy particles. For example, the followingmethods are known: a method of coating starting material particles suchas acicular goethite particles and acicular hematite particles with acobalt compound in advance and thereafter heat-treating the particles ina reducing atmosphere [Japanese Patent Application Laid-Open (KOKAI) No.54-122664 (1979) and Japanese Patent Publication No. 58-55203 (1983)]; amethod of coating starting material particles with a boron compound inadvance and thereafter heat-treating the particles in a reducingatmosphere [Japanese Patent Application Laid-Open (KOKAI) No. 57-57459(1982), Japanese Patent Publication No. 54-42832 (1979), Japanese PatentApplication Laid-Open (KOKAI) Nos. 58-48611 (1983), 58-46607 (1983),59-32881 (1984), 59-5603 (1984), 61-174304 (1986), 61-186410 (1986) andJapanese Patent Publication No. 59-32881 (1984 )]; and a method ofcoating starting material particles with a water soluble boron compoundand the water-soluble salts of Al, Cr, Ge, Nd and the like in advance,and thereafter heat-treating the particles in a reducing atmosphere[Japanese Patent Application Laid-Open (KOKAI) No. 61-186410 (1986)].

Acicular magnetic iron based alloy particles having a high coerciveforce, a large saturation magnetization, an excellent stability againstoxidation and an excellent S.F.D. are now in the strongest demand, butno acicular magnetic iron based alloy particles having all theseproperties together have been produced by any known method.

More specifically, although acicular magnetic iron based alloy particleshaving a high coercive force and a large saturation magnetization aregenerally obtained by the above-described known methods, for example,the stability of saturation magnetization of the acicular magnetic ironbased alloy particles produced by the process described in JapanesePatent Application Laid-Open (KOKAI) No. 58.46607 (1983) is about 10 to30%, and the stability of saturation magnetization of the acicularmagnetic iron based alloy particles produced by the process described inJapanese Patent Application Laid-Open (KOKAI) No. 59-5603 (1984) isabout 15%, both being insufficient. The S.F.D. of the conventionalacicular magnetic iron based alloy particles are also unsatisfactory aswill be described later.

Accordingly, acicular magnetic iron based alloy particles which have ahigh coercive force, a high saturation magnetization, an excellentstability against oxidation and an excellent S.F.D., and further have agood dispersibility ascribed to excellent wetting property anddeagglomeration property with respect to a binder, in particular, abinder containing a resin having an acid functional group, or a largespecific surface area in addition to the above properties are stronglydemanded.

As a result of studies undertaken by the present inventors in order tomeet such demand, it has been found that acicular magnetic iron basedalloy particles containing 1.5 to 10 mol % of B based on Fe (calculatedas B) and 1.5 to 10 mol % of Co based on Fe (calculated as Co) in thevicinity of the surfaces of the particles which are produced by coatingthe surfaces of acicular goethite particles with a boron compound and acobalt compound, heat-treating the thus obtained particles in atemperature range of 300° to 600° C. to obtain acicular hematiteparticles coated with a boron oxide and a cobalt oxide, andheat-treating the thus obtained acicular hematite particles in atemperature range of 300° to 500° C. in a reducing atmosphere, have ahigh coercive force, a large saturation magnetization, an excellentstability against oxidation and an excellent S.F.D.

It has also been found that by coating the surfaces of the aciculargoethite particles further with an aluminum compound and/or a nickelcompound so that 1.0 to 10 mol % of Al based on Fe (calculated as Al)and/or 0.3 to 10 mol % of Ni based on Fe (calculated as Ni) in additionto B and Co described above is contained in the vicinity of the surfacesof the acicular magnetic iron based alloy particles, the thus obtainedacicular magnetic iron based alloy particles have a good dispersibilityascribed to excellent wetting property and deagglomeration property withrespect to a binder, in particular, a binder containing a resin havingan acid functional group in addition to the above properties.

It has also been found that by coating the surfaces of the aciculargoethite particles further with a silicon compound so that Si in amountof 1.0 to 10 mol % of SiO₂ based on Fe (calculated as SiO₂) in additionto B and Co described above are contained in the vicinity of thesurfaces of the acicular magnetic iron based alloy particles, the thusobtained acicular magnetic iron based alloy particles obtained have alarge specific surface area in addition to the above properties.

The present invention has been made on the basis of these findings.

SUMMARY OF THE INVENTION

In a first aspect of the present invention, there is provided acicularmagnetic iron based alloy particles for magnetic recording, containing1.5 to 10 mol % of B based on Fe (calculated as B) and 1.5 to 10 mol %of Co based on Fe (calculated as Co) in the vicinity of the surfaces ofsaid particles and having a saturation magnetization of not less than125 emu/g and an S.F.D. value of not more than 0.50.

In a second aspect of the present invention, there is provided acicularmagnetic iron based alloy particles for magnetic recording comprisingacicular magnetic iron based alloy particles containing 1.5 to 10 mol %of B based on Fe (calculated as B), 1.5 to 10 mol % of Co based on Fe(calculated as Co) and 1.0 to 10 mol % of Al based on Fe (calculated asAl) and/or 0.3 to 10 mol % of Ni based on Fe (calculated as Ni) in thevicinity of the surfaces of said particles and having a saturationmagnetization of not less than 125 emu/g and an S.F.D. value of not morethan 0.50.

In a third aspect of the present invention, there is provided acicularmagnetic iron based alloy particles for magnetic recording comprisingacicular magnetic iron based alloy particles containing 1.5 to 10 mol %of B based on Fe (calculated as B), 1.5 to 10 mol % of Co based on Fe(calculated as Co) and Si in amount of 1.0 to 10 mol % of SiO₂ based onFe (calculated as SiO₂) or Si in amount of 1.0 to 10 mol % of SiO₂ basedon Fe (calculated as SiO₂) and 1.0 to 10 mol % of Al based on Fe(calculated as Al) and having a saturation magnetization of not lessthan 125 emu/g and an S.F.D. value of not more than 0.50.

In a fourth aspect of the present invention, there is provided a processfor producing acicular magnetic iron based alloy particles for magneticrecording containing boron and cobalt in the vicinity of the surfacesthereof and having a saturation magnetization of not less than 125 emu/gand an S.F.D. value of not more than 0.50, comprising the steps of:coating the surfaces of acicular goethite particles with a boroncompound and a cobalt compound; heat-treating said particles in atemperature range of 300° to 600° C. to obtain acicular hematiteparticles coated with a boron oxide and a cobalt oxide; andheat-treating said acicular hematite particles in a temperature range of300° to 500° C. in a reducing atmosphere.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(1) Acicular magnetic iron based alloy particles according to thepresent invention which contain 1.5 to 10 mol % of B based on Fe(calculated as B) and 1.5 to 10 mol % of Co based on Fe (calculated asCo) in the vicinity of the surfaces of the particles, have a highcoercive force of not less than 1,400 Oe, a large saturationmagnetization of not less than 125 emu/g, and a change of saturationmagnetization of not more than 15%, preferably not more than 10%, and asmall S.F.D. value of not more than 0.50.

(2) Acicular magnetic iron based alloy particles according to thepresent invention which contain 1.5 to 10 mol % of B based on Fe(calculated as B), 1.5 to 10 mol % of Co based on Fe (calculated as Co)and 1.0 to 10 mol % of Al based on Fe (calculated as Al) and/or 0.3 to10 mol % of Ni based on Fe (calculated as Ni) in the vicinity of thesurfaces of the particles, have a high coercive force of not less than1,400 Oe, a large saturation magnetization of not less than 125 emu/g,and a change of saturation magnetization of not more than 15%,preferably not more than 10%, a small S.F.D. value of not more than0.50, and a good dispersibility ascribed to excellent wetting propertyand deagglomeration property with respect to a binder, in particular, abinder containing a resin having an acid functional group.

(3) Acicular magnetic iron based alloy particles according to thepresent invention which contain 1.5 to 10 mol % of B based on Fe(calculated as B), 1.5 to 10 mol % of Co based on Fe (calculated as Co)and Si in amount of 1.0 to 10 mol % of SiO₂ based on Fe (calculated asSiO₂), or Si in amount of 1.0 to 10 mol % of SiO₂ Si based on Fe(calculated as SiO₂) and 1.0 to 10 mol % of Al based on Fe (calculatedas Al) in the vicinity of the surfaces thereof, have a high coerciveforce of not less than 1,400 Oe, a large saturation magnetization of notless than 125 emu/g, and a change of saturation magnetization of notmore than 15%, preferably not more than 10%, a small S.F.D. value of notmore than 0.50, and a large specific surface area of not less than 45.0m² /g.

Acicular magnetic iron based alloy particles according to the presentinvention are produced by coating the surfaces of acicular goethiteparticles with:

(1) a boron compound and a cobalt compound;

(2) a boron compound, a cobalt compound, and an aluminum compound and/ora nickel compound; or

(3) a boron compound, a cobalt compound and a silicon compound, or asilicon compound and an aluminum compound,

heat-treating the thus obtained particles in a temperature range of 300°to 600° C. to obtain coated acicular hematite particles, andheat-treating the thus obtained acicular hematite particles in atemperature range of 300° to 500° C. in a reducing atmosphere.

(1) Acicular magnetic iron based alloy particles which is an object ofthe present invention, having a high coercive force, a large saturationmagnetization, an excellent stability against oxidation and an excellentS.F.D. cannot be obtained either in the case of coating the startingmaterial particles only with a boron compound or a cobalt compound, orin the case of introducing boron an/or cobalt elements into the startingmaterial particles, as will be shown in the later-described comparativeexamples. That is, it is only by coating the starting material particleswith a boron compound and a cobalt compound that acicular magnetic ironbased alloy particles according to the present invention are obtained.

(2) Acicular magnetic iron based alloy particles according to thepresent invention containing aluminum in addition to boron and cobalt inthe vicinity of the surfaces of the particles have a large resinadsorption, as will be shown in the later-described examples, so that itis clear that these particles have an excellent wetting property with abinder containing a resin, in particular, a resin having an acidfunctional group in addition to the properties described in (1).

Acicular magnetic iron based alloy particles according to the presentinvention containing nickel in addition to boron and cobalt in thevicinity of the surfaces of the particles are excellent in the initialdispersibility, as will be shown in the later-described examples, sothat it is clear that these particles have an excellent deagglomerationproperty with a binder containing a resin, in particular, a resin havingan acid functional group in addition to the properties described in (1).

Acicular magnetic iron based alloy particles according to the presentinvention containing aluminum and nickel in addition to boron and cobaltin the vicinity of the surfaces of the particles are excellent in thegloss, as will be shown in the later-described examples, so that it isclear that these particles have an excellent dispersibility ascribed toexcellent wetting property and deagglomeration property with a bindercontaining a resin, in particular, a resin having an acid functionalgroup in addition to the properties described in (1).

(3) Acicular magnetic iron based alloy particles according to thepresent invention containing boron, cobalt and silicon, or silicon andaluminum in the vicinity of the surfaces of the particles have a largespecific surface area in addition to the properties of the particlescontaining boron and cobalt in the vicinity of the surfaces thereofdescribed in (1).

The acicular goethite particles used in the present invention can beproduced by any ordinary method such as a method of oxidizing asuspension containing ferrous hydroxide and having a pH of not less than11 obtained by mixing an aqueous ferrous salt solution and not less thanan equivalent of an alkali solution, by passing an oxygen-containing gasthereinto at a temperature of not higher than 80° C. [Japanese PatentPublication No. 39-5610 (1964)] and a method of oxidizing a suspensioncontaining FeCO₃ obtained by reacting an aqueous ferrous salt solutionwith an alkali carbonate, by passing an oxygen-containing gas thereintoJapanese Patent Application Laid Open (KOKAI) No. 50-80999 (1975)]. Bychanging reaction conditions, additives, etc. in these method, it ispossible to use particles having a particle length of 0.1 to 0.4 μm andan aspect ratio (major axis:minor axis) of 5:1 to 20:1. In the producingreaction of the goethite particles, the metal ions of Co, Ni, Zn, Mn,Cu, etc. which are generally added in order to improve the properties ofthe acicular magnetic iron based alloy particles may be added.

In order to obtain acicular magnetic iron based alloy particles having apreferable S.F.D. value, especially, an S.F.D. value of not more than0.47, the acicular goethite particles having a uniform particle size andbeing free of dendrites which are obtained by the latter method are usedas the starting material particles. In particular, when aciculargoethite particles having a uniform particle size, being free ofdendrites, having a particle length of 0.18 to 0.3 μm and a large aspectratio (major axis:minor axis), especially an aspect ratio of not lessthan 10:1 are used, it is possible to obtain acicular magnetic ironbased alloy particles having a more preferable S.F.D. value especiallyan S.F.D. value of not more than 0.45.

As the acicular goethite particles of the starting material of thepresent invention, those disclosed in U.S. Pat. Nos. 4,133,677,4,130,158, 4,437,881, 3,652,334 and 4,773,931, Japanese PatentPublications No. 55-29577 (1980), No. 60-11446 (1985) and No. 59-48768(1984), Japanese Patent Application No. 63-325436 (1988) and U.S. patentapplication Ser. No. 07/342152 are usable.

As the boron compound in the present invention, KBO₂, H₃ BO₃, HBO₂, B₂O₃, etc. are usable. The coating of acicular goethite particles with aboron compound is carried out by mixing an aqueous solution containingboron with the acicular goethite particles, stirring the mixture,filtering out the particles and drying the thus obtained particles. Theamount of boron coating the particle surfaces is 1.5 to 10 mol % basedon Fe (calculated as B). If it is less than 1.5 mol %, sintering ofparticle and sintering between particles is caused, thereby making itdifficult to obtain acicular magnetic iron based alloy particles havinga high coercive force. If it is more than 10 mol %, the progress ofreduction is obstructed, thereby making it difficult to obtain acicularmagnetic iron based alloy particles which are objective particles of thepresent invention.

As the cobalt compound in the present invention, cobalt sulfate, cobaltnitrate, cobalt acetate, cobalt chloride, cobalt hydroxide, etc. areusable. The coating of acicular goethite particles with a cobaltcompound is carried out by mixing an aqueous solution containing cobaltwith the acicular goethite particles, stirring the mixture, neutralizingthe mixture with an aqueous alkaline solution or an aqueous acidsolution, if necessary, filtering out the particles and drying the thusobtained particles. The amount of cobalt coating the particle surfacesis 1.5 to 10.0 mol % based on Fe (calculated as Co). If it is less than1.5 mol %, the saturation magnetization is not more than 125 emu/g andthe S.F.D. value exceeds 0.50, thereby making is difficult to obtain theacicular magnetic iron based alloy particles which are objectiveparticles of the present invention. Even if it is more than 10.0 mol %,it is possible to obtain the acicular magnetic iron based alloyparticles which are objective particles of the present invention, but itis meaningless that the acicular magnetic iron based alloy particlescontain Co more than necessary.

As the aluminum compound in the present invention, aluminum sulfate,aluminum nitrate, aluminum chloride, aluminum hydroxide, sodiumaluminate, alumina sol and the like are usable. The coating of aciculargoethite particles with an aluminum compound is carried out by mixing anaqueous solution containing aluminum with the acicular goethiteparticles, stirring the mixture, neutralizing the mixture with anaqueous alkaline solution or an aqueous acid solution, if necessary,filtering out the particles and drying the thus obtained particles. Theamount of aluminum coating the particle surfaces is 1.0 to 10.0 mol %,preferably 1.0 to 5 mol % based on Fe (calculated as Al). If it is lessthan 1.0 mol %, the wetting property with a resin is sufficient, therebymaking it difficult to obtain acicular magnetic iron based alloyparticles having a sufficient resin adsorption, or it is difficult toobtain acicular magnetic iron based alloy particles having a largespecific surface area. If it is more than 10.0 mol %, the magneticcharacteristics such as the coercive force and the saturationmagnetization of the acicular magnetic iron based alloy particlesobtained are greatly reduced.

As the nickel compound in the present invention, nickel sulfate, nickelnitrate, nickel chloride, nickel hydroxide, etc. are usable. The coatingof acicular goethite particles with a nickel compound is carried out bymixing an aqueous solution containing nickel with the acicular goethiteparticles, stirring the mixture, neutralizing the mixture with anaqueous alkaline solution or an aqueous acid solution, if necessary,filtering out the particles and drying the thus obtained particles. Theamount of nickel coating the particle surfaces is 0.3 to 10.0 mol %based on Fe (calculated as Ni). If it is less than 0.3 mol %, thereducing-temperature lowering-effect is not obtained, and since thedeagglomeration property of the particles is insufficient, theimprovement of the initial dispersibility, which is one of the objectsof the present invention, is insufficient. If it is more than 10.0 mol%, the magnetic crystalline anisotropy become smaller, thereby making itdifficult to obtain acicular magnetic iron based alloy particles havinga high coercive force, and the saturation magnetization is apt to belowered.

As the silicon compound in the present invention, colloidal silica,water glass, aqueous silicic acid, silica sol, etc. are usable. Thecoating of acicular goethite particles with a silicon compound iscarried-out by mixing an aqueous solution containing a silicon compoundwith the acicular goethite particles, stirring the mixture, neutralizingthe mixture with an aqueous alkaline solution or an aqueous acidsolution, if necessary, filtering out the particles and drying the thusobtained particles. The amount of silicon coating the particle surfacesis 1.0 to 10.0 mol % based on Fe (calculated as SiO₂). If it is lessthan 1.0 mol %, it is difficult to obtain the iron based alloy particleshaving a large specific surface area. If it is more than 10.0 mol %, thesaturation magnetization of the acicular magnetic iron based alloyparticles obtained is unfavorably lowered.

When the surfaces of acicular goethite particles are coated with a boroncompound, cobalt compound, aluminum compound, nickel compound, andsilicon compound, the order of coating may be selected as desired, orcoating of such compounds may be carried out simultaneously.

The heat-treating temperature in the present invention is 300° to 600°C. If it is lower than 300° C., since high densification of theparticles is difficult, it is difficult to retain the shapes of theparticles at the time of heat-treating in the subsequent reductionprocess, thereby lowering the coercive force of the acicular magneticiron based alloy particles, and whereby it is also difficult to providean S.F.D. value of not more than 0.5. The reason why the S.F.D. value isnot improved is that when the heat-treating temperature is lower than300° C., recrystallization is insufficient, resulting in thenonuniformity in the shape anisotropy due to the distribution of theshapes of the particles after reduction and the nonuniformity in thecrystalline anisotropy due to the structural distribution of Fe, B andCo. If the heat-treating temperature is higher than 600° C., sinteringof particle and sintering between particles is caused, therebycollapsing the shapes of the particles.

The heat-treating temperature in a reducing atmosphere in the presentinvention is 300° to 500° C. If it is lower than 300° C., the reductionis insufficient for obtaining acicular magnetic iron based alloyparticles having a large saturation magnetization. If it is higher than500° C., sintering of particle and sintering between particles iscaused, thereby collapsing the shapes of the particles and lowering thecoercive force of the acicular magnetic iron based alloy particlesobtained.

The acicular magnetic iron based alloy particles according to thepresent invention has a particle length of 0.1 to 0.4 μm, and an aspectratio (major axis:minor axis) of 5:1 to 15:1.

The acicular magnetic iron based alloy particles after reduction of thepresent invention can be taken out into air by a known method such as amethod of dipping the acicular magnetic iron based alloy particles in anorganic solvent such as toluene and a method of replacing the atmosphereafter reduction with an inert gas and gradually increasing the oxygencontent in the inert gas so as to finally achieving the gradualoxidization of the innert gas by air.

The acicular magnetic iron based alloy particles according to thepresent invention obtained in this way have:

(1) a coercive force not less than 1,400 Oe, preferably not less than1450 Oe; a saturation magnetization of not less than 125 emu/g,preferably not less than 130 emu/g; a change of saturation magnetizationof not more than 15%, preferably not more than 10%; and an S.F.D. valueof not more than 0.50, preferably not more than 0.47;

(2) a gloss of not less than 100%, preferably not less than 110% at adispersion time of 50%; a gloss of not less than 120%; preferably notless than 130% at a dispersion time of 100%; and a resin adsorption ofnot less than 1.1 g, preferably not less than 1.4 g; in addition to theproperties described in (1); or

(3) a specific surface area of not less than 45.0 m² /g, preferably notless than 48 m² /g in addition to the properties described in (1).

The acicular magnetic iron based alloy particles of the presentinvention having the above-described properties are suitable as themagnetic particles for high-density and high-output recording which arenow in the strongest demand.

The present invention will be more precisely explained while referringto Examples as follows.

However, the present invention is not restricted to Examples undermentioned. From the foregoing description, one skilled in the art caneasily ascertain the essential characteristics of this invention, andwithout departing from the spirit and scope thereof, can make variouschanges and modifications of the invention to adapt it to various usagesand conditions.

EXAMPLES

In each of the following Examples and Comparative Examples;

(i) The particle length and the aspect ratio (major axis:minor axis) ofthe acicular magnetic iron based alloy particles are expressed by theaverage values measured in electron microphotographs.

(ii) The magnetic characteristics of the acicular magnetic iron basedalloy particles were measured by using a vibrating sample magnetometerVSM-3S-15, produced by Toei Kogyo K.K. and applying an external magneticfield up to 10 KOe.

(iii) The stability against oxidation is expressed by Δσs/σs (%) whichshows the changes of the saturation magnetization (%) of the sampleallowed to stand for 7 days at a temperature of 60° C. and a relativehumidity of 90%.

(iv) The S.F.D. value was obtained by using a sheet test piece having asquareness ratio of 0.85 obtained by the following method. Thedifferentiation curve of the magnetization major roop around coercivityof the sample was obtained by using a differentiation circuit of themagnetometer and the half power width of the differentiation curve wasmeasured and the half power width was divided by the coercive force ofthe peak value of the curve.

A magnetic coating was prepared by charging the following componentsinto a 100-cc plastic bottle in the following ratio and dispersing themwith a paint conditioner for 8 hours. The sheet test piece was obtainedby applying the magnetic coating to polyethylene film of 25 μm thick toa thickness of 50 μm by using an applicator, and drying the coated filmin a magnetic field of 3 to 5 KGauss.

    ______________________________________                                        3-mmφ steel ball                                                                             800    parts by weight                                     Magnetic iron based                                                                              100    parts by weight                                     alloy particles                                                               Polyurethane resin 20     parts by weight                                     having a sodium                                                               sulfonate group                                                               Cyclohexanone      83.3   parts by weight                                     Methylethyl ketone 83.3   parts by weight                                     Toluene            83.3   parts by weight                                     ______________________________________                                    

(v) The resin adsorption is expressed by the difference between theweight of the resin concentration of the resin liquid having thefollowing composition and the weight of the resin concentration of thesupernatant liquid of the coating obtained by charging 20 g of acicularmagnetic iron based alloy particles, 56 g of the resin liquid and 120 gof 3-mmφ stainless steel balls into a 100-ml plastic bottle anddispersing them with a paint conditioner for 60 minutes and subjected tocentrifugal separation. The weights of both resin concentrations weremeasured as the nonvolatile matter.

    ______________________________________                                        Composition of resin liquid                                                   ______________________________________                                        Solids content of resin                                                                             3.57   wt %                                             MEK                   32.14  wt %                                             Toluene               32.14  wt %                                             Anone                 32.14  wt %                                             ______________________________________                                    

(vi) The dispersibility is expressed by the gloss of the coated filmobtained by applying a coating to PET film by an applicator. The coatingwas produced by charging 15 g of acicular magnetic iron based alloyparticles, the resin liquid having the following composition and 120 gof 3-mmφ stainless steel balls into a 100-ml plastic bottle anddispersing them with a paint conditioner for a predetermined time. Thedispersing time required for the saturation of the gloss was assumed tobe 100%, and the gloss at that time is expressed as the criterion of thedispersibility. The gloss at a dispersion time of 50% is expressed asthe criterion of the initial dispersibility.

    ______________________________________                                        Composition of resin liquid                                                   ______________________________________                                        Resin                 3.57   g                                                MEK                   15.17  g                                                Toluene               15.17  g                                                Cyclohexanone         15.17  g                                                ______________________________________                                    

The gloss of the coated film was measured by a glossmeter produced byNihon Denshoku Kogyo K.K. at an incident angle of 60° C., and the valueis expressed by % on the assumption that the gloss of the standard sheetwas 89.0%.

Production of acicular hematite particles Examples 1 to 10, ComparativeExamples 1 to 9 Example 1

100 g of acicular goethite particles having a particle length of 0.21 μmand a aspect ratio (major axis:minor axis) of 12:1 was suspended in 1 lof water.

To the suspension, 10.0 g (equivalent to 10.0 wt % based on the aciculargoethite particles) of H₃ BO₃ and 13.0 g (equivalent to 13.0 wt % basedon the acicular goethite particles) of Co(CH₃ COO)₂.4H₂ O were added andthe suspension was stirred for 10 minutes. The pH of the suspension was6.2. NH₄ OH was then added until the pH of the suspension became 9.3.The acicular α-FeOOH particles were filtered out and dried, therebyobtaining the acicular α-FeOOH particles coated with boron and cobalt.

50 g of the thus obtained acicular goethite particles coated with boronand cobalt were heat-treated at 400° C. in air to obtain acicularhematite particles coated with a boron oxide and a cobalt oxide.According to the observation of the electron microphotographs, theparticles had an average particle length of 0.19 μm and an averageaspect ratio (major axis:minor axis) of 11:1.

Examples 2 to 10, Comparative Examples 1 to 9

Acicular hematite particles were obtained in the same way as in Example1 except that the kind of the acicular goethite particles, the kind andthe content of a boron compound and the time for mixing the boroncompound, the kind and the content of a cobalt compound and the time ofexistence of the cobalt compound and the temperature in theheat-treating process were varied.

The main conditions for producing these particles and the properties ofthese particles obtained are shown in Table 1.

Production of acicular magnetic iron based alloy particles Examples 11to 20, Comparative Examples 10 to 20 Example 11

20 g of the acicular hematite particles coated with boron oxide andcobalt oxide obtained in Example 1 were reduced by a hydrogen gas at410° C. for 6 hours, thereby obtaining acicular magnetic iron basedalloy particles.

The surfaces of the acicular magnetic iron based alloy particlesobtained by reduction were coated with a stable oxide film so as toprevent rapid oxidation when they were taken out into air. As a resultof X-ray fluorometry, it was found that the particles contained 4.7 mol% of B based on Fe and 4.5 mol % of Co based on Fe. According to theobservation of the electron microphotographs, the particles had anaverage particle length of 0.15 μm and an average aspect ratio (majoraxis:minor axis) of 7:1. As to the magnetic characteristics, thecoercive force (Hc) was 1,530 Oe, the saturation magnetization (σs) was133 emu/g, the of saturation magnetization was 8.0% and the S.F.D. valuewas 0.43.

Examples 12 to 20, Comparative Examples 10 to 20

Acicular magnetic iron based alloy particles were obtained in the sameway as in Example 11 except that the kind of the acicular hematiteparticles, and the time and the temperature in the heat-treating processwere varied.

The main conditions for producing these particles and the properties ofthese particles obtained are shown in Table 2.

Surface coating of acicular goethite particles

Examples 21 to 39, Reference Examples 1 to 4

Example 21

100 g of acicular goethite particles having an average particle lengthof 0.21 μm and an average aspect ratio (major axis:minor axis) of 12:1was suspended in 1 l of water.

To the suspension, 10.0 g (equivalent to 10.0 wt % based on the aciculargoethite particles) of H₃ BO₃, 13.0 g (equivalent to 13.0 wt % based onthe acicular goethite particles) of Co(CH₃ COO)₂.4H₂ O and 12 g(equivalent to 12 wt % based on the acicular goethite particles) ofAl(NO₃)₃.9H₂ O were added and the suspension was stirred for 10 minutes.The pH of the suspension was 4.4. NH₄ OH was then added until the pH ofthe suspension became 9.3. The acicular α-FeOOH particles were filteredout and dried, thereby obtaining the acicular α-FeOOH particles coatedwith boron compound, cobalt compound and aluminum compound.

Examples 22 to 39, Reference Examples 1 to 4

Acicular α-FeOOH particles coated with various compounds were obtainedin the same way as in Example 21 except that the kind of the aciculargoethite particles, the kind and the content of a boron compound, thekind and the content of a cobalt compound and the kind and the contentof an aluminum compound or a nickel compound or the kinds and thecontents of both alumina and nickel compounds were varied.

The main conditions for producing these particles and the properties ofthese particles obtained are shown in Tables 3 and 4.

Production of acicular hematite particles Examples 40 to 58, ComparativeExamples 5 to 8 Example 40

50 g of the acicular goethite particles coated with boron, cobalt andaluminum obtained in Example 21 were heat-treated at 400° C. in air toobtain acicular hematite particles coated with a boron oxide, a cobaltoxide and an aluminum oxide. According to the observation of theelectron microphotographs, the particles had an average particle lengthof 0.19 μm and an average aspect ratio (major axis:minor axis) of 11:1by average value.

Examples 41 to 58, Reference Examples 5 to 8

Acicular hematite particles were obtained in the same way as in Example40 except that the temperature in the heat-treating process was varied.

The main conditions for producing these particles and the properties ofthese particles obtained are shown in Tables 5 and 6.

Production of Acicular Magnetic Iron Based Alloy Particles Examples 59to 77, Reference Examples 9 to 12 Example 59

20 g of the acicular hematite particles coated with a boron oxide, acobalt oxide and an aluminum oxide obtained in Example 40 were reducedby a hydrogen gas at 420° C. for 6 hours, thereby obtaining acicularmagnetic iron based alloy particles.

The surfaces of the acicular magnetic iron based alloy particlesobtained by reduction were coated with a stable oxide film so as toprevent rapid oxidation when they were taken out into air. As a resultof X-ray fluorometry, it was found that the particles contained 4.7 mol% of B based on Fe, 4.5 mol % of Co based on Fe and 2.6 mol % of Albased on Fe. According to the observation of the electronmicrophotographs, the particles had an average particle length of 0.15μm and an average aspect ratio (major axis:minor axis) of 7:1. As to themagnetic characteristics, the coercive force (Hc) was 1,540 Oe, thesaturation magnetization (σs) was 133 emu/g, the oxidative stability was8.0% and the S.F.D. value was 0.42. The resin adsorption content was1.4. As to the dispersibility, the gloss at a dispersion time of 50% was110% and the gloss at a dispersion time of 100% was 120%.

Examples 60 to 77, Reference Examples 9 to 12

Acicular magnetic iron based alloy particles were obtained in the sameway as in Example 59 except that the kind of the acicular hematiteparticles and the temperature in the heat-treating process were varied.

The main conditions for producing these particles and the properties ofthese particles obtained are shown in Tables 7 and 8.

Surface coating of acicular goethite particles Examples 78 to 90,Reference Examples 13 to 14 Example 78

100 g of acicular goethite particles having an average particle lengthof 0.20 μm and an average aspect ratio (major axis:minor axis) of 13:1was suspended in 1 l of water.

To the suspension, 10.0 g (equivalent to 10.0 wt % based on the aciculargoethite particles) of H₃ BO₃, 13.0 g (equivalent to 13.0 wt % based onthe acicular goethite particles) of Co(CH₃ COO)₂.4H₂ O and 9.5 g(equivalent to 9.5 wt % based on the acicular goethite particles) ofcolloidal silica were added and the suspension was stirred for 10minutes. The pH of the suspension was 6.1. NH₄ OH was then added untilthe pH of the suspension became 9.3. The acicular α-FeOOH particles werefiltered out and dried, thereby obtaining the acicular α-FeOOH particlescoated with boron compound, cobalt compound and silicon compound.

Examples 79 to 90, Reference Example 13 and 14

Acicular α-FeOOH particles coated with various compounds were obtainedin the same way as in Example 78 except that the kind of the aciculargoethite particles, the kind and the content of a boron compound, thekind and the content of a cobalt compound, the kind and the content of asilicon compound and the kind or the content of an aluminum compound, ifany, were varied.

The main conditions for producing these particles and the properties ofthese particles obtained are shown in Tables 9 and 10.

In the case of using water glass as the silicon compound for coating theacicular goethite particles, after the water glass was added to thesuspension with the pH adjusted to 9.3, the suspension was stirred for10 minutes, and then acetic acid was added to adjust the pH of thesuspension to 7.0 to 7.5.

Production of acicular hematite particles Examples 91 to 105,Comparative Examples 15 to 16 Example 91

50 g of the acicular goethite particles coated with boron compound,cobalt compound and silicon compound obtained in Example 78 wereheat-treated at 400° C. in air to obtain acicular hematite particlescoated with a boron oxide, a cobalt oxide and a silicon oxide. Accordingto the observation of the electron microphotographs, the particles hadan average particle length of 0.19 μm and an average aspect ratio (majoraxis:minor axis) of 11:1.

Examples 92 to 105, Reference Examples 15 and 16

Acicular hematite particles were obtained in the same way as in Example91 except that the temperature in the heat-treating process was varied.

The main conditions for producing these particles and the properties ofthese particles obtained are shown in Tables 11 and 12.

Production of Acicular Magnetic Iron Based Alloy Particles Examples 106to 120, Reference Examples 17 and 18 Example 106

20 g of the acicular hematite particles coated with a boron oxide, acobalt oxide and an aluminum oxide obtained in Example 91 were reducedby a hydrogen gas at 410° C. for 6 hours, thereby obtaining acicularmagnetic iron based alloy particles.

The surfaces of the acicular magnetic iron based alloy particlesobtained by reduction were coated with a stable oxide film so as toprevent rapid oxidation when they were taken out into air. As a resultof X-ray fluorometry, it was found that the particles contained 4.7 mol% of B based on Fe, 4.5 mol % of Co based on Fe and 2.4 mol % of Sibased on Fe (calculated as SiO₂). According to the observation of theelectron microphotographs, the particles proved to have an averageparticle length of 0.15 μm and an average aspect ratio (major axis:minoraxis) of 7:1. As to the magnetic characteristics, the coercive force(Hc) was 1,560 Oe, the saturation magnetization (σs) was 133 emu/g, thechanges of saturation magnetization was 8.0% and the S.F.D. was 0.43.

Examples 107 to 120, Comparative Examples 17 and 18

Acicular magnetic iron based alloy particles were obtained in the sameway as in Example 106 except that the kind of the acicular hematiteparticles and the temperature in the heat-treating process were varied.

The main conditions for producing these particles and the properties ofthese particles obtained are shown in Tables 13 and 14.

                                      TABLE 1                                     __________________________________________________________________________    Examples                                                                             Acicular goethite particles                                            and                Aspect ratio                                                                           Boron compound                                                                              Cobalt compound                     Comparative                                                                          Process                                                                            Particle                                                                             (Major axis:Minor                                                                          Content                                                                            Existence       Content                                                                            Existence           Examples                                                                             (*)  length (μm)                                                                       axis)    Kind                                                                              (wt %)                                                                             (**) Kind       (wt                                                                                (**)                __________________________________________________________________________    Example 1                                                                            A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 2                                                                            A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          22.0 (a)                 Example 3                                                                            A    0.21   12:1     H.sub.3 BO.sub.3                                                                  20   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 4                                                                            A    0.21   12:1     B.sub.2 O.sub.3                                                                   5.5  (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 5                                                                            A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(NO.sub.3).sub.2.6H.sub.2                                                              14.9 (a)                 Example 6                                                                            A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 7                                                                            A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 8                                                                            A    0.15    9:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 9                                                                            B    0.15   10:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 10                                                                           B    0.22   13:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Comparative                                                                          A    0.21   12:1     H.sub.3 BO.sub.3                                                                  2.5  (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 1                                                                     Comparative                                                                          A    0.21   12:1     --  --   --   Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 2                                                                     Comparative                                                                          A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          2.8  (a)                 Example 3                                                                     Comparative                                                                          A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10   (a)  --         --   --                  Example 4                                                                     Comparative                                                                          A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 5                                                                     Comparative                                                                          A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 6                                                                     Comparative                                                                          A    0.20   11:1     H.sub.3 BO.sub.3                                                                  10   (a)  Co(CH.sub.3 COO).sub.                                                         2.4H.sub.2 O                                                                             13.0 (b)                 Example 7                                                                     Comparative                                                                          A    0.22   13:1     H.sub.3 BO.sub.3                                                                  10   (b)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (a)                 Example 8                                                                     Comparative                                                                          A    0.22   12:1     H.sub.3 BO.sub.3                                                                  10   (b)  Co(CH.sub.3 COO).sub.2.4H.sub.2                                               O          13.0 (b)                 Example 9                                                                     __________________________________________________________________________                           Examples and         Acicular hematite particles                              Comparative Heat-treatment                                                                         Particle                                                                             Aspect ratio                                      Examples    temperature (°C.)                                                               length (μm)                                                                       (Major axis:Minor          __________________________________________________________________________                                                       axis)                                             Example 1   400      0.19   11:1                                              Example 2   400      0.19   11:1                                              Example 3   400      0.19   11:1                                              Example 4   400      0.19   11:1                                              Example 5   400      0.19   11:1                                              Example 6   350      0.20   11:1                                              Example 7   500      0.19   10:1                                              Example 8   400      0.14    8:1                                              Example 9   400      0.12    8:1                                              Example 10  400      0.19   11:1                                              Comparative Example 1                                                                     400      0.17   11:1                                              Comparative Example 2                                                                     400      0.17   10:1                                              Comparative Example 3                                                                     400      0.19   11:1                                              Comparative Example 4                                                                     400      0.19   11:1                                              Comparative Example 5                                                                     650      0.12    9:1                                              Comparative Example 6                                                                     250      0.21   12:1                                              Comparative Example 7                                                                     400      0.18   10:1                                              Comparative Example 8                                                                     400      0.17   11:1                                              Comparative Example 9                                                                     400      0.17   11:1                       __________________________________________________________________________     (Note)                                                                        (*) Process A: Aqueous sodium carbonate is used as aqueous alkali             solution.                                                                     Process B: Aqueous sodium hydroxide is used as aqueous alkali solution.       (**) (a): Existent on the surfaces of acicular goethite particles.            (b): Existent in aqueous ferrous sulfate.                                

                                      TABLE 2                                     __________________________________________________________________________           Kind of acicular     Acicular magnetic iron                                   hematite particle                                                                      Heat-Treatment in                                                                         based alloy particles                             Examples and                                                                         (Example No. and                                                                       reducing atmosphere   Coercive                                                                           Saturation                         Comparative                                                                          Comparative                                                                            Temperature                                                                          Time B/Fe Co/Fe                                                                              force He                                                                           magnetization                      Examples                                                                             Example No.)                                                                           (°C.)                                                                         (Hour)                                                                             (mol %)                                                                            (mol %)                                                                            (Oe) σs (emu/g)                   __________________________________________________________________________    Example 11                                                                           Example 1                                                                              410    6    4.7  4.5  1530 133                                Example 12                                                                           Example 2                                                                              390    6    4.9  7.3  1590 135                                Example 13                                                                           Example 3                                                                              420    6    8.3  4.6  1550 131                                Example 14                                                                           Example 4                                                                              410    6    4.6  4.5  1520 134                                Example 15                                                                           Example 5                                                                              410    6    4.7  4.4  1530 133                                Example 16                                                                           Example 6                                                                              410    6    4.7  4.5  1510 130                                Example 17                                                                           Example 7                                                                              410    6    4.7  4.5  1540 134                                Example 18                                                                           Example 8                                                                              410    6    4.7  4.5  1550 135                                Example 19                                                                           Example 9                                                                              410    6    4.7  4.5  1550 134                                Example 20                                                                           Example 10                                                                             410    6    4.7  4.5  1570 136                                Comparative                                                                          Example 1                                                                              310    6    4.7  4.5  1210 112                                Example 10                                                                    Comparative                                                                          Example 2                                                                              510    6    4.7  4.5  1180 137                                Example 11                                                                    Comparative                                                                          Comparative                                                                            390    6    1.2  4.4  1100 138                                Example 12                                                                           Example 1                                                              Comparative                                                                          Comparative                                                                            380    6    --   4.4   890 140                                Example 13                                                                           Example 2                                                              Comparative                                                                          Comparative                                                                            410    6    4.6  1.0  1480 123                                Example 14                                                                           Example 3                                                              Comparative                                                                          Comparative                                                                            420    6    4.5  --   1450 120                                Example 15                                                                           Example 4                                                              Comparative                                                                          Comparative                                                                            410    6    4.7  4.5  1240 132                                Example 16                                                                           Example 5                                                              Comparative                                                                          Comparative                                                                            410    6    4.7  4.5  1370 123                                Example 17                                                                           Example 6                                                              Comparative                                                                          Comparative                                                                            410    6    4.7  4.5  1540 132                                Example 18                                                                           Example 7                                                              Comparative                                                                          Comparative                                                                            390    6    1.0  4.5  1100 138                                Example 19                                                                           Example 8                                                              Comparative                                                                          Comparative                                                                            390    6    1.1  4.5  1110 138                                Example 20                                                                           Example 9                                                              __________________________________________________________________________                     Acicular magnetic iron based alloy particles                                  Change of                                                    Examples and     saturation   Particle                                                                             Aspect ratio                             Comparative Examples                                                                           magnetization (%)                                                                      S.F.D.                                                                            length (μm)                                                                       (Major axis:Minor axis)                  __________________________________________________________________________    Example 11       8.0      0.43                                                                              0.15   7:1                                      Example 12       8.0      0.42                                                                              0.14   6:1                                      Example 13       8.0      0.43                                                                              0.16   7:1                                      Example 14       8.0      0.43                                                                              0.15   7:1                                      Example 15       7.0      0.43                                                                              0.15   7:1                                      Example 16       7.0      0.44                                                                              0.15   7:1                                      Example 17       8.0      0.43                                                                              0.15   7:1                                      Example 18       8.0      0.46                                                                              0.10   6:1                                      Example 19       8.0      0.49                                                                              0.10   6:1                                      Example 20       9.0      0.48                                                                              0.16   9:1                                      Comparative Example 10                                                                         5.0      0.42                                                                              0.15   7:1                                      Comparative Example 11                                                                         15.0     0.65                                                                              0.07   4:1                                      Comparative Example 12                                                                         11.0     0.57                                                                              0.06   3:1                                      Comparative Example 13                                                                         10.0     0.66                                                                              0.05   3:1                                      Comparative Example 14                                                                         12.0     0.56                                                                              0.16   7:1                                      Comparative Example 15                                                                         14.0     0.58                                                                              0.16   7:1                                      Comparative Example 16                                                                         15.0     0.64                                                                              0.08   5:1                                      Comparative Example 17                                                                         11.0     0.53                                                                              0.14   6:1                                      Comparative Example 18                                                                         15.0     0.54                                                                              0.15   7:1                                      Comparative Example 19                                                                         13.0     0.61                                                                              0.06   3:1                                      Comparative Example 20                                                                         18.0     0.60                                                                              0.07   3:1                                      __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Acicular goethite particles                                                                    Aspect ratio                                                                           Boron compound                                                                         Cobalt compound                                 Process                                                                            Particle                                                                             (Major axis:Minor                                                                          Content         Content                         Examples                                                                           (*)  length (μm)                                                                       axis)    Kind                                                                              (wt %)                                                                             Kind       (wt %)                          __________________________________________________________________________    21   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            22   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            23   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            24   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            25   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            26   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            27   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            28   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            29   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub. 2.4H.sub.2                                                         13.0                            30   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            31   A    0.21   12:1     B.sub.2 O.sub.3                                                                   5.5  Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            32   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(NO.sub.3).sub.2.6H.sub.2 O                                                            14.9                            33   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            34   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            35   A    0.15    9:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            36   B    0.15   10:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            37   B    0.22   13:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            38   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  20.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                            39   A    0.21   12:1     H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          22.0                            __________________________________________________________________________                          Aluminum compound                                                                          Nickel compound                                                          Content         Content                                          Examples                                                                           Kind    (wt %)                                                                             Kind       (wt %)                          __________________________________________________________________________                     21   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   --         --                                               22   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         18   --         --                                               23   Alumina sol                                                                           8.1  --         --                                               24   Alumina sol                                                                           22.4 --         --                                               25   --      --   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              26   --      --   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          15.0                                             27   --      --   Ni(NO.sub.3).sub.2.6H.sub.2 O                                                            1.7                                              28   --      --   Ni(NO.sub.3).sub.2.6H.sub.2 O                                                            17.3                                             29   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              30   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         18   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              31   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              32   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              33   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              34   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              35   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              36   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              37   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              38   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                                              39   Al(NO.sub.3).sub.3.9H.sub.2 O                                                         12   Ni(CH.sub.3 COO).sub.2.4H.sub.2                                                          1.4                             __________________________________________________________________________     (Note)                                                                        (*) Process A: Aqueous sodium carbonate is used as aqueous alkali             solution.                                                                     Process B: Aqueous sodium hydroxide is used as aqueous alkali solution.  

                                      TABLE 4                                     __________________________________________________________________________                       Boron compound                                                                         Cobalt compound                                   Reference                                                                           Acicular goethite particles                                                                    Content         Content                                Example                                                                             Kind         Kind                                                                              (wt %)                                                                             Kind       (wt %)                                 __________________________________________________________________________    1     The same acicular goethite                                                                 H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                        13.0                                         particles as those used in                                                    Example 21.                                                             2     The same acicular goethite                                                                 H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                        13.0                                         particles as those used in                                                    Example 21.                                                             3     The same acicular goethite                                                                 H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                        13.0                                         particles as those used in                                                    Example 21.                                                             4     The same acicular goethite                                                                 H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                        13.0                                         particles as those used in                                                    Example 21.                                                             __________________________________________________________________________                   Aluminum compound                                                                         Nickel compound                                             Reference     Content         Content                                         Example                                                                             Kind    (wt %)                                                                             Kind       (wt %)                                 __________________________________________________________________________             1     Al(NO.sub.3).sub.3.9H.sub.2 O                                                         2.5  --         --                                              2     Al(NO.sub.3).sub.3.9H.sub.2 O                                                         47.0 --         --                                              3     Al(NO.sub. 3).sub.3.9H.sub.2 O                                                        --   Ni(CH.sub.3 COO).sub.2.4H.sub.2 O                                                        0.3                                             4     Al(NO.sub.3).sub.3.9H.sub.2 O                                                         --   Ni(CH.sub.3 COO).sub.2.4H.sub.2 O                                                        34.0                                   __________________________________________________________________________

                  TABLE 5                                                         ______________________________________                                                             Acicular hematite                                                            particles                                                                                      Aspect                                          Kind of acicular                                                                          Heat-             ratio                                           goethite    treatment  Particle                                                                             (Major                                          particles   Temperature                                                                              length axis:Minor                               Examples                                                                             (Example No.)                                                                             (°C.)                                                                             (μm)                                                                              axis)                                    ______________________________________                                        40     21          400        0.19   11:1                                     41     22          400        0.19   11:1                                     42     23          400        0.19   11:1                                     43     24          400        0.19   11:1                                     44     25          400        0.19   11:1                                     45     26          400        0.19   11:1                                     46     27          400        0.19   11:1                                     47     28          400        0.19   11:1                                     48     29          400        0.19   11:1                                     49     30          400        0.19   11:1                                     50     31          400        0.19   11:1                                     51     32          400        0.19   11:1                                     52     33          350        0.20   11:1                                     53     34          500        0.19   10:1                                     54     35          400        0.14    8:1                                     55     36          400        0.12    8:1                                     56     37          400        0.19   11:1                                     57     38          400        0.19   11:1                                     58     39          400        0.19   11:1                                     ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                                             Acicular hematite                                                      Heat-  particles                                                        Kind of acicular                                                                          treatment       Aspect ratio                                      goethite particles                                                                        Tempera- Particle                                                                             (Major                                    Reference                                                                             (Reference  ture     length axis:Minor                                Examples                                                                              Example No.)                                                                              (°C.)                                                                           (μm)                                                                              axis)                                     ______________________________________                                        5       1           400      0.19   11:1                                      6       2           400      0.19   11:1                                      7       3           400      0.19   11:1                                      8       4           400      0.19   11:1                                      ______________________________________                                    

                                      TABLE 7                                     __________________________________________________________________________                            Acicular magnetic iron based alloy particles          Kind of acicular                                                                           Heat-treatment in                          Change of             hematite     reducing atmosphere            Coercive                                                                           Saturation                                                                           saturation                 particles                                                                             Temperature                                                                          Time                                                                              B/Fe Co/Fe                                                                              Al/Fe                                                                              Ni/Fe                                                                              force Hc                                                                           magnetization                                                                        magnetization         Examples                                                                           (Example No.)                                                                         (°C.)                                                                         (hour)                                                                            (mol %)                                                                            (mol %)                                                                            (mol %)                                                                            (mol %)                                                                            (Oe) σs                                                                             (%)u/g)               __________________________________________________________________________    59   40      420    6   4.7  4.5  2.6  --   1540 133    8.0                   60   41      440    6   4.8  4.5  3.9  --   1520 131    8.0                   61   42      410    6   4.7  4.5  2.6  --   1530 133    8.0                   62   43      410    6   4.7  4.5  7.2  --   1530 132    7.0                   63   44      400    6   4.7  4.5  --   0.5  1540 134    8.0                   64   45      390    6   4.8  4.5  --   5.1  1530 133    8.0                   65   46      400    6   4.7  4.5  --   0.5  1540 135    8.0                   66   47      390    6   4.7  4.5  --   5.0  1520 133    8.0                   67   48      410    6   4.7  4.5  2.6  0.5  1540 134    8.0                   68   49      420    6   4.7  4.5  3.9  0.5  1520 132    8.0                   69   50      410    6   4.6  4.5  2.6  0.5  1540 133    8.0                   70   51      410    6   4.7  4.4  2.6  0.5  1530 132    8.0                   71   52      410    6   4.7  4.5  2.6  0.5  1500 130    7.0                   72   53      410    6   4.7  4.5  2.6  0.5  1550 135    8.0                   73   54      410    6   4.7  4.5  2.6  0.5  1550 135    8.0                   74   55      410    6   4.7  4.5  2.6  0.5  1550 134    8.0                   75   56      410    6   4.7  4.5  2.6  0.5  1560 136    9.0                   76   57      410    6   8.3  4.6  2.6  0.5  1570 131    8.0                   77   58      410    6   4.9  7.3  2.6  0.5  1600 138    7.0                   __________________________________________________________________________                       Acicular magnetic iron based alloy particle                                       Gloss (%)                                                                     50% disper-                                                                         100% disper-                                                                         Adsorbed resin                                                                        Particle                                                                             Aspect ratio                             Examples                                                                           S.F.D.                                                                            sion Time                                                                           sion Time                                                                            content (g)                                                                           length (μm)                                                                       (Major axis:Minor          __________________________________________________________________________                                                       axis)                                    59   0.42                                                                              110   120    1.4     0.15   7:1                                      60   0.40                                                                              120   130    1.5     0.15   7:1                                      61   0.42                                                                              110   120    1.4     0.15   7:1                                      62   0.40                                                                              120   130    1.6     0.15   7:1                                      63   0.43                                                                              100   120    1.1     0.15   7:1                                      64   0.42                                                                              120   130    1.1     0.14   6:1                                      65   0.43                                                                              100   120    1.1     0.15   7:1                                      66   0.43                                                                              120   130    1.1     0.14   6:1                                      67   0.42                                                                              120   130    1.4     0.15   7:1                                      68   0.41                                                                              130   140    1.5     0.16   7:1                                      69   0.42                                                                              120   130    1.4     0.15   7:1                                      70   0.42                                                                              120   130    1.4     0.15   7:1                                      71   0.42                                                                              120   130    1.4     0.16   7:1                                      72   0.42                                                                              120   130    1.4     0.15   7:1                                      73   0.45                                                                              120   130    1.4     0.10   6:1                                      74   0.48                                                                              120   130    1.4     0.10   6:1                                      75   0.47                                                                              120   130    1.4     0.16   9:1                                      76   0.42                                                                              120   130    1.4     0.16   7:1                                      77   0.39                                                                              120   130    1.4     0.14   6:1                        __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    Kind of acicular        Acicular magnetic iron based alloy particles               hematite                                                                              Heat-treatment in                          Change of             Re-  particles                                                                             reducing atmosphere            Coercive                                                                           Saturation                                                                           saturation            ference                                                                            (Reference                                                                            Temperature                                                                          Time                                                                              B/Fe Co/Fe                                                                              Al/Fe                                                                              Ni/Fe                                                                              force Hc                                                                           magnetization                                                                        magnetization         Examples                                                                           Example No.)                                                                          (°C.)                                                                         (hour)                                                                            (mol %)                                                                            (mol %)                                                                            (mol %)                                                                            (mol %)                                                                            (Oe) σs                                                                             (%)u/g)               __________________________________________________________________________    9    5       410    6   4.7  4.5  0.5  --   1530 133    8.0                   10   6       450    6   4.7  4.5  10.3 --   1400 121    6.0                   11   7       410    6   4.7  4.5  --   0.1  1530 133    8.0                   12   8       380    6   4.7  4.5  --   12.0 1210 112    7.0                   __________________________________________________________________________                       Acicular magnetic iron based alloy particles                                      Gloss (%)                                                           Reference 50% disper-                                                                         100% disper-                                                                         Adsorbed resin                                                                        Particle                                                                             Aspect ratio                            Examples                                                                            S.F.D.                                                                            sion Time                                                                           sion Time                                                                            content (g)                                                                           length (μm)                                                                       (Major axis:Minor          __________________________________________________________________________                                                       axis)                                   9     0.43                                                                              70    100    1.1     0.15   7:1                                     10    0.49                                                                              90    120    1.7     0.14   6:1                                     11    0.43                                                                              70    100    1.1     0.15   7:1                                     12    0.55                                                                              90    110    1.1     0.13   6:1                        __________________________________________________________________________

                                      TABLE 9                                     __________________________________________________________________________    Acicular goethite particles                                                                    Aspect ratio                                                                           Boron compound                                                                          Cobalt compound                                Process                                                                            Particle                                                                             (Major axis:Minor                                                                           Content         Content                        Examples                                                                           (*)  length (μm)                                                                       axis)    Kind (wt %)                                                                             Kind       (wt %)                         __________________________________________________________________________    78   A    0.20   13:1     H.sub.3 BO.sub.3                                                                   10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                           79   A    0.20   13:1     H.sub.3 BO.sub.3                                                                   10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                           80   A    0.20   13:1     H.sub.3 BO.sub.3                                                                   10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                           81   A    0.20   13:1     H.sub.3 BO.sub.3                                                                   10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                           82   A    0.20   13:1     H.sub.3 BO.sub.3                                                                   10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                           83   A    0.20   13:1     H.sub.3 BO.sub.3                                                                   10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                           84   A    0.20   13:1     B.sub.2 O.sub.3                                                                    5.5  Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                           85   A    0.20   13:1     H.sub.3 BO.sub.3                                                                   10.0 Co(NO.sub.3).sub.2.6H.sub.2 O                                                            14.9                           86   A    0.15   10:1     H.sub.3 BO.sub.3                                                                   10.0 Co(CH.sub.3 COO).sub.2 .4H.sub.2                                                         13.0                           87   B    0.15   11:1     H.sub.3 BO.sub.3                                                                   10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                           88   B    0.23   13:1     H.sub.3 BO.sub.3                                                                   10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                           89   A    0.20   13:1     H.sub.3 BO.sub.3                                                                   20.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          13.0                           90   A    0.20   13:1     H.sub.3 BO.sub.3                                                                   10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2                                                          22.0                           __________________________________________________________________________                              Silicon compound                                                                           Aluminum compound                                                        Content      Content                                             Examples                                                                           Kind    (wt %)                                                                             Kind    (wt %)                         __________________________________________________________________________                         78   Colloidal silica                                                                      9.5  --      --                                                  79   Colloidal silica                                                                      29.0 --      --                                                  80   Colloidal silica                                                                      19.0 Al(NO.sub.3).sub.3.9H.sub.2                                                           12.0                                                81   Colloidal silica                                                                      29.0 Alumina sol                                                                           8.1                                                 82   Colloidal silica                                                                      19.0 Alumina sol                                                                           22.4                                                83   Water glass                                                                           14.0 Alumina sol                                                                           8.1                                                 84   Colloidal silica                                                                      19.0 Al(NO.sub.3).sub.3.9H.sub.2                                                           12.0                                                85   Colloidal silica                                                                      19.0 Al(NO.sub.3).sub.3.9H.sub.2                                                           12.0                                                86   Colloidal silica                                                                      19.0 Al(NO.sub.3).sub.3.9H.sub.2                                                           12.0                                                87   Colloidal silica                                                                      19.0 Al(NO.sub.3).sub.3.9H.sub.2                                                           12.0                                                88   Colloidal silica                                                                      19.0 Al(NO.sub.3).sub.3.9H.sub.2                                                           12.0                                                89   Colloidal silica                                                                      19.0 Al(NO.sub.3).sub.3.9H.sub.2                                                           12.0                                                90   Colloidal silica                                                                      19.0 Al(NO.sub.3).sub.3.9H.sub.2                                                           12.0                           __________________________________________________________________________     (Note)                                                                        (*) Process A: Aqueous sodium carbonate is used as aqueous alkali             solution.                                                                     Process B: Aqueous sodium hydroxide is used as aqueous alkali solution.  

                                      TABLE 10                                    __________________________________________________________________________                       Boron compound                                                                         Cobalt compound                                   Reference                                                                           Acicular goethite particles                                                                    Content         Content                                Examples                                                                            Kind         Kind                                                                              (wt %)                                                                             Kind       (wt %)                                 __________________________________________________________________________    13    The same acicular goethite                                                                 H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                        13.0                                         particles as those used in                                                    Example 78.                                                             14    The same acicular goethite                                                                 H.sub.3 BO.sub.3                                                                  10.0 Co(CH.sub.3 COO).sub.2.4H.sub.2 O                                                        13.0                                         particles as those used in                                                    Example 78.                                                             __________________________________________________________________________                   Silicon compound                                                                          Aluminum compound                                           Reference     Content         Content                                         Examples                                                                            Kind    (wt %)                                                                             Kind       (wt %)                                 __________________________________________________________________________             13    Colloidal silica                                                                      1.8  Al(NO.sub.3).sub.3.9H.sub.2 O                                                            12.0                                            14    Colloidal silica                                                                      48.0 Al(NO.sub.3).sub.3.9H.sub.2 O                                                            12.0                                   __________________________________________________________________________

                  TABLE 11                                                        ______________________________________                                                             Acicular hematite                                                            particles                                                                                      Aspect                                          Kind of acicular                                                                          Heat-             ratio                                           goethite    treatment  Particle                                                                             (Major                                          particles   Temperature                                                                              length axis:Minor                               Examples                                                                             (Example No.)                                                                             (°C.)                                                                             (μm)                                                                              axis)                                    ______________________________________                                        91     78          400        0.19   11:1                                     92     79          400        0.19   11:1                                     93     80          400        0.19   11:1                                     94     80          350        0.20   11:1                                     95     80          500        0.19   10:1                                     96     81          400        0.19   11:1                                     97     82          400        0.19   11:1                                     98     83          400        0.19   11:1                                     99     84          400        0.19   11:1                                     100    85          400        0.19   11:1                                     101    86          400        0.14    8:1                                     102    87          400        0.12    8:1                                     103    88          400        0.19   11:1                                     104    89          400        0.19   11:1                                     105    90          400        0.19   11:1                                     ______________________________________                                    

                  TABLE 12                                                        ______________________________________                                                             Acicular hematite                                                      Heat-  particles                                                        Kind of acicular                                                                          treatment       Aspect ratio                                      goethite particles                                                                        Tempera- Particle                                                                             (Major                                    Reference                                                                             (Reference  ture     length axis:Minor                                Examples                                                                              Example No.)                                                                              (°C.)                                                                           (μm)                                                                              axis)                                     ______________________________________                                        15      13          400      0.19   11:1                                      16      14          400      0.19   11:1                                      ______________________________________                                    

                                      TABLE 13                                    __________________________________________________________________________                            Acicular magnetic iron based alloy particles          Kind of acicular                                                                           Heat-treatment in                          Change of             hematite     reducing atmosphere            Coercive                                                                           Saturation                                                                           saturation                 particles                                                                             Temperature                                                                          Time                                                                              B/Fe Co/Fe                                                                              SiO.sub.2 /Fe                                                                      Al/Fe                                                                              force Hc                                                                           magnetization                                                                        magnetization         Examples                                                                           (Example No.)                                                                         (°C.)                                                                         (hour)                                                                            (mol %)                                                                            (mol %)                                                                            (mol %)                                                                            (mol %)                                                                            (Oe) σs                                                                             (%)u/g)               __________________________________________________________________________    106  91      410    6   4.7  4.5  2.4  --   1560 133    8.0                   107  92      430    6   4.7  4.5  6.9  --   1620 131    5.0                   108  93      430    6   4.7  4.5  4.7  2.6  1580 133    7.0                   109  94      410    6   4.7  4.5  4.7  2.6  1540 130    6.0                   110  95      410    6   4.7  4.5  4.7  2.6  1590 135    7.0                   111  96      420    6   4.7  4.5  6.9  2.6  1610 130    5.0                   112  97      410    6   4.7  4.5  4.7  7.2  1570 132    7.0                   113  98      410    6   4.7  4.5  4.6  2.6  1580 133    7.0                   114  99      410    6   4.6  4.5  4.7  2.6  1580 132    7.0                   115  100     410    6   4.7  4.4  4.7  2.6  1570 135    7.0                   116  101     410    6   4.7  4.5  4.7  2.6  1590 135    7.0                   117  102     410    6   4.7  4.5  4.7  2.6  1590 134    7.0                   118  103     410    6   4.7  4.5  4.7  2.6  1600 135    8.0                   119  104     410    6   8.3  4.6  4.7  2.6  1610 131    8.0                   120  105     410    6   4.9  7.3  4.7  2.6  1640 138    6.0                   __________________________________________________________________________                                    Acicular magnetic iron based alloy                                            particles                                                                         Specific surface                                                                      Particle                                                                             Aspect ratio                                          Examples                                                                           S.F.D.                                                                            area (m.sup.2 /g)                                                                     length (μm)                                                                       (Major axis:Minor          __________________________________________________________________________                                                       axis)                                                 106  0.43                                                                              49.8    0.15   7:1                                                   107  0.44                                                                              53.3    0.15   7:1                                                   108  0.42                                                                              51.6    0.15   7:1                                                   109  0.42                                                                              52.0    0.16   7:1                                                   110  0.42                                                                              51.0    0.15   7:1                                                   111  0.42                                                                              55.0    0.15   7:1                                                   112  0.40                                                                              57.6    0.15   7:1                                                   113  0.42                                                                              51.1    0.15   7:1                                                   114  0.42                                                                              51.3    0.15   7:1                                                   115  0.41                                                                              51.1    0.15   7:1                                                   116  0.45                                                                              49.9    0.10   6:1                                                   117  0.48                                                                              51.3    0.10   6:1                                                   118  0.47                                                                              54.5    0.16   9:1                                                   119  0.42                                                                              51.0    0.16   7:1                                                   120  0.40                                                                              50.9    0.14   6:1                        __________________________________________________________________________

                                      TABLE 14                                    __________________________________________________________________________    Kind of acicular        Acicular magnetic iron based alloy particles               hematite                                                                              Heat-treatment in                          Change of             Re-  particles                                                                             reducing atmosphere            Coercive                                                                           Saturation                                                                           saturation            ference                                                                            (Reference                                                                            Temperature                                                                          Time                                                                              B/Fe Co/Fe                                                                              SiO.sub.2 /Fe                                                                      Al/Fe                                                                              force Hc                                                                           magnetization                                                                        magnetization         Examples                                                                           Example No.)                                                                          (°C.)                                                                         (hour)                                                                            (mol %)                                                                            (mol %)                                                                            (mol %)                                                                            (mol %)                                                                            (Oe) σs                                                                             (%)u/g)               __________________________________________________________________________    17   15      410    6   4.7  4.5  0.5  2.6  1530 133    8.0                   18   16      450    6   4.7  4.5  11.4 2.6  1600 98     5.0                   __________________________________________________________________________                                    Acicular magnetic iron based alloy                                            particles                                                               Reference Specific surface                                                                      Particle                                                                             Aspect ratio                                         Examples                                                                            S.F.D.                                                                            area (m.sup.2 /g)                                                                     length (μm)                                                                       (Major axis:Minor          __________________________________________________________________________                                                       axis)                                                17    0.43                                                                              43.2    0.15   7:1                                                  18    0.56                                                                              62.4    0.16   8:1                        __________________________________________________________________________

What is claimed is:
 1. Acicular magnetic iron based alloy particles formagnetic recording, each having 1.5 to 10 mol % of B based on Fe and 1.5to 10 mol % of Co based on Fe close to the particle surfaces, and havinga saturation magnetization of not less than 125 emu/g, an S.F.D. valueof not more than 0.50 and a coercive force of not less than 1450 Oe, achange of saturation magnetization (Δσs/σs) of not more than 15%, and aparticle length of 0.1 to 0.4 μm and an aspect ratio (major axis:minoraxis) of 5:1 to 15:1.
 2. Acicular magnetic iron based alloy particlesfor magnetic recording according to claim 1 which additionally have 1.0to 10 mol % of Al based on Fe close to the particle surfaces. 3.Acicular magnetic iron based alloy particles for magnetic recordingaccording to claim 1, which additionally have 0.4 to 10 mol % of Nibased on Fe close to the particle surfaces.
 4. Acicular magnetic ironbased alloy particles for magnetic recording according to claim 1 whichadditionally have each 1.0 to 10 mol % of Al based on Fe and 0.3 to 10mol % of Ni based on Fe close to the particle surfaces.
 5. Acicularmagnetic iron based alloy particles for magnetic recording according toclaim 1, which additionally have 1.0 to 10 mol % of Si based on Fe(calculated as SiO₂) close to the particle surfaces.
 6. Acicularmagnetic iron based alloy particles for magnetic recording according toclaim 1, which additionally have 1.0 to 10 mol % of Si based on Fe(calculated as SiO₂) and 1.0 to 10 mol % of Al based on Fe close to theparticle surfaces.
 7. Acicular magnetic iron based alloy particles formagnetic recording according to claim 2, 3 or 4, which have a gloss ofnot less than 100% at a dispersion time of 50% and a gloss of not lessthan 120% at a dispersion time of 100%.
 8. Acicular magnetic iron basedalloy particles for magnetic recording according to claim 2, 3 or 4,which have a resin adsorption of not less than 1.1 g based on 20 g ofthe acicular magnetic iron based alloy particles.
 9. Acicular magneticiron based alloy particles for magnetic recording according to claim 5or 6, which have a specific surface area of not less than 45 m² /g. 10.Acicular magnetic iron based alloy particles for magnetic recordingaccording to claim 1, which are produced by:(a) coating the surfaces ofacicular goethite particles with a boron compound and a cobalt compound;(b) heat-treating said particles at a temperature in the range of 300°to 600° C. to obtain acicular hematite coated with a boron oxide and acobalt oxide; and (c) heat-treating said acicular hematite particles ata temperature in the range of 300° to 500° C. in a reducing atmosphere.